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Issa says Attorney General Eric Holder has blocked an investigation into "Operation Fast and Furious"

Issa accuses Holder of incompetence and says Holder misled Congress about what he knew and when he knew it

Fast and Furious is a discredited federal gunrunning operation designed to track weapons to drug cartels

A top House Republican charged with investigating a now-discredited federal gunrunning operation has accused Attorney General Eric Holder of actively obstructing Congress' oversight function and damaging his own credibility as a top national law enforcement officer.

In a letter to Holder publicly released Monday, House Oversight and Government Reform Committee Chairman Darrell Issa, R-California, said the attorney general has made "numerous statements" about Operation Fast and Furious that "have eventually been proven to be untrue."

Issa indicated in an interview over the weekend on "Fox News Sunday" that he could issue subpoenas related to the politically sensitive matter to the Justice Department as soon as this week.

"The time for deflecting blame and obstructing our investigation is over," Issa wrote in the letter, which was dated Sunday. "The time has come for you to come clean to the American public about what you knew about Fast and Furious, when you knew it, and who is going to be held accountable for failing to shut down a program that has already had deadly consequences, and will likely cause more casualties for years to come."

Issa blasted Holder for "negligence and incompetence" on the issue, and for offering a "roving set of ever-changing explanations" designed primarily to "circle the wagons around (the Justice Department) and its political appointees."

The operation was the Justice Department's "most significant gun trafficking case," Issa said. "On your watch, it went spectacularly wrong. Whether you realize yet or not, you own Fast and Furious. It is your responsibility."

Operation Fast and Furious involved agents from the Bureau of Alcohol, Tobacco, Firearms and Explosives allowing illegal sales of guns believed to be destined for Mexican drug cartels to "walk" from Phoenix, Arizona, gun stores into Mexico.

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The idea was to track the sellers and purchasers of guns to Mexican cartels, but the program became mired in controversy after weapons found at Mexican and American murder scenes were traced back to the program. Mexican officials and critics in the United States called the program a failure, saying it exacerbated the longstanding problem of U.S. weapons getting into the hands of the violent Mexican cartels.

Holder testified before the Judiciary Committee in May that he had known about the Fast and Furious program for just a few weeks. Republicans insist recently released Justice Department documents show the attorney general actually knew about the program much earlier.

Holder and his aides continue to vehemently deny that charge.

The attorney general responded angrily Friday to GOP critics of his handling of the operation, charging them with using "irresponsible and inflammatory rhetoric."

"I simply cannot sit idly by as a (Republican) member of the House Committee on Oversight and Government Reform suggests, as happened this week, that law enforcement and government employees who devote their lives to protecting our citizens be considered 'accessories to murder,'" Holder said in a letter to members of Congress.

Such rhetoric, Holder declared, "must be repudiated in the strongest possible terms."

Asked for a reaction to Issa's letter, DOJ spokeswoman Tracy Schmaler said Monday that "these recycled allegations continue to be baseless, no matter how many times they are repeated."

The Justice Department will cooperate with congressional investigations, but "what the American people deserve is less partisan showboating and more responsible solutions to stopping gun violence on the southwest border," the official said.

On Sunday, Issa said the Judiciary Committee has invited Holder to "come and clear the record."

"Clearly, he knew when he said he didn't know," Issa said. "Now the question is what did he know and how is he going to explain why he gave that answer."